This invention relates to semiconductor devices and manufacturing processes therefor, and more particularly to reduction of resistance in narrow structures in FET devices.
In the manufacture of FET devices with polysilicon gates, the ongoing reduction in size of the gates has led to increased resistance of the gate conductor. To overcome this problem, a silicide layer (typically CoSi2) is often placed on top of the polysilicon. The addition of the silicide to the narrow polysilicon gate conductor has been shown to be effective in reducing the resistance. In addition, it is known that forming a CoSi2 layer on other active regions (the source and drain, adjacent to the gate) has a beneficial effect by reducing sheet resistance in those regions. Processes for forming the silicide in a self-aligned fashion are known in the art.
Formation of a cobalt silicide on the gate conductor typically begins with deposition of a Co layer on top of the polysilicon. To prevent oxidation of the cobalt, a capping layer is generally used. Titanium nitride (TiN) is an effective capping layer, serving as an oxygen barrier with good adhesion to the cobalt, while not reacting with the underlying silicon. A typical silicide formation process is shown in FIGS. 1A to 1C. A standard precleaning is performed before deposition to minimize native oxide on the Si surface. As shown in FIG. 1A, the silicon gate structure 1 has a cobalt layer 2 deposited thereon; the thickness of the Co layer 2 is about 80 xc3x85.
A TiN capping layer 3 is then deposited on the cobalt; this conventionally is done by sputtering a Ti target using Ar atoms, in a nitrogen atmosphere. Those skilled in the art will appreciate that as the N2 flow is varied, the target voltage (the voltage between the Ti target and ground) may be measured as a dependent variable. A graph of this relationship is shown in FIG. 2. Details of the plot of target voltage vs. N2 flow will, of course, vary with process conditions and between sputtering tools; the voltages and flowrates shown in FIG. 2 are examples only. There are three distinct regions of N2 flow, indicated by I, II and III in FIG. 2:
I. xe2x80x9cNon-nitridedxe2x80x9d target: too little N2 is present in the sputtering chamber to form a nitride on the Ti target. Accordingly, pure Ti is sputtered from the target. The Ti may react with N in the chamber to form TixNy (that is, non-stoichiometric TiN) or Ti(N) (that is, Ti with N in solution). The Co surface thus is coated either with pure Ti (which may later react to form TiN), TixNy or Ti(N).
II. Transition region: this region is characterized by plasma instabilities, and is avoided in practice because of the difficulty of controlling the sputtering process.
III. xe2x80x9cNitridedxe2x80x9d target: sufficient N2 is present to form a nitride, TixNy, on the target. The stoichiometry of the nitride formed on the target depends on the N2 flow actually used. Sputtering this TiN results in grains of TiN being formed on the Si surface; the nitrogen content of the TiN may be further enhanced by the inclusion of N atoms at the grain boundaries. A conventional TiN layer is produced by sputtering in this region of N2 flow.
It follows that TiN produced with N2 flow in region III may be characterized as xe2x80x9cN-richxe2x80x9d while TiN produced with N2 flow in region I may be characterized as xe2x80x9cN-deficient.xe2x80x9d
In the conventional TiN capping layer process, the thickness of the TiN layer is about 200 xc3x85. The Si/Co/TiN structure is then annealed in an inert atmosphere (often N2 or Ar) at a temperature approximately in the range 480xc2x0 C. to 570xc2x0 C., preferably about 540xc2x0 C. This annealing step causes the cobalt to react with the silicon to produce a layer 4 of cobalt silicide, CoSi, in place of the Co layer (FIG. 1B). If the Co layer 2 is about 80 xc3x85 thick, the thickness of the CoSi layer is generally 200 xc3x85 to 300 xc3x85 thick. The TiN capping layer may then be stripped away (typically using a sulfuric acid-hydrogen peroxide mixture). A second anneal, at a temperature approximately in the range 680xc2x0 C. to 750xc2x0 C., results in formation of a layer 5 of cobalt disilicide on the Si gate 1 (FIG. 1C). The CoSi2 is a low resistance conductor and has a thickness slightly greater than of the CoSi layer (300 xc3x85 to 400 xc3x85 in this example).
Even though a preclean is performed, in practice the surface of the silicon 1 is covered by a native oxide with a thickness typically about 5 xc3x85 to 10 xc3x85. This oxide is shown as layer 11 in FIG. 3A. When the Co layer 2 is deposited on top of the oxide 11, Co and Si atoms diffuse toward each other through the oxide, as shown schematically in FIG. 3B. After the first anneal, layers of CoSi 12, 13 are formed above and below the oxide, respectively (FIG. 3C; compare with the idealized picture in FIG. 1B). The thin native oxide does not interfere with the reaction between the Co and the Si to form the silicide.
As noted above, the conventional TiN in capping layer 3 is generally not truly stoichiometric, but includes additional nitrogen. Nitrogen atoms may thus diffuse out of the capping layer 3 into and through the cobalt layer 2. In addition, N may be incorporated in the Co layer or at the Co/TiN interface during deposition of the capping layer. Although possible beneficial effects of introducing nitrogen into a self-aligned CoSi2 are known (for example, improving thermal stability to agglomeration), the involvement of nitrogen in the cobalt silicide formation process has an undesirable effect. Specifically, diffusion of N atoms from the TiN capping layer 3 to the oxide layer 11 (see FIG. 4A) may result in formation of an oxynitride layer 21, which blocks diffusion of Si atoms 10 to the cobalt layer 2 (FIG. 4B; compare FIG. 3B). A thick oxynitride may also inhibit transport of Co atoms. This results in incomplete formation of the CoSi, with a layer 22 of unreacted Co above the oxynitride 21 after the first anneal (FIG. 4C; compare FIG. 3C). This Co layer 22 is stripped away with the TiN capping layer 3, leaving a thin layer of CoSi. This in turn results in a thin layer 25 of CoSi2 being formed in the second anneal (FIG. 4D). Discontinuities in the CoSi2 layer 25 (that is, incomplete coverage of the Si gate 1) have been observed.
There is therefore a need for a capping layer for the cobalt metal which in general controls the introduction of N into the cobalt prior to formation of the CoSi2, and in particular avoids formation of an oxynitride between the cobalt and silicon, thereby permitting complete formation of the CoSi
The present invention addresses the above-described need by providing a capping layer for the silicide-forming metal such that nitrogen diffusion therefrom is insufficient to cause formation of an oxynitride from the oxide layer on the silicon.
According to a first aspect of the invention, the capping layer is a metal layer overlying the semiconductor structure and in contact with the silicide-forming metal; this metal layer is composed of tungsten, molybdenum, tantalum or another refractory metal. If the layer is of tungsten, the thickness thereof is approximately in the range 25 xc3x85 to 150 xc3x85.
According to another aspect of the invention, the capping layer is a layer of nitride, such as titanium nitride (TiN), overlying the semiconductor structure and in contact with the silicide-forming metal, where the layer has a nitrogen content such that diffusion of nitrogen from that layer through the silicide-forming metal is prevented. Specifically, this nitride layer may be non-stoichiometric TiN deficient in nitrogen. Accordingly, the diffusion of nitrogen from the nitride layer is insufficient to cause formation of an oxynitride at the oxide layer on the silicon surface.
According to another aspect of the invention, the capping layer has a first layer in contact with the silicide-forming metal, and a second layer overlying the first layer and in contact therewith; the second layer has a composition distinct from that of the first layer. Each of these layers preferably includes titanium. Specifically, the first layer may be titanium nitride (TiN) including a first amount of nitrogen, while the second layer is TiN including a second amount of nitrogen greater than the first amount. The first and second layers each have a thickness of approximately 100 xc3x85. These distinct TiN layers may be produced by sputtering Ti in the presence of N2 flows in regions I and III respectively, thereby producing N-deficient and N-rich TiN layers. Alternatively, the first layer may consist essentially of Ti and with a thickness not greater than about 20 xc3x85, while the second layer comprises TiN having a thickness of approximately 200 xc3x85. As another alternative, the second layer may consist essentially of Ti with a thickness of approximately 200 xc3x85, while the first layer comprises TiN. to a further aspect of the invention, a method is provided for forming a capping layer for a semiconductor structure, where the semiconductor structure includes a silicide-forming metal overlying silicon. The method includes the step of forming a layer of nitride overlying the semiconductor structure and in contact with the silicide-forming metal; the layer has a nitrogen content such that diffusion of nitrogen from the layer through the silicide-forming metal is prevented. The layer of nitride is preferably titanium nitride. The forming step may include sputtering from a titanium target in an ambient characterized by a nitrogen flow, where the nitrogen flow is insufficient to cause formation of a nitride on the target; accordingly, the layer of nitride comprises non-stoichiometric TiN deficient in nitrogen.
According to an additional aspect of the invention, the method may include the steps of forming a first layer and a second layer, with the first layer and second layer having distinct compositions. The step of forming the first layer may include sputtering from a titanium target in an ambient characterized by a nitrogen flow, where the nitrogen flow is insufficient to cause formation of a nitride on the target; accordingly, the first layer may be non-stoichiometric TiN deficient in nitrogen. Alternatively, the step of forming the first layer may include sputtering from a titanium target to produce a layer consisting essentially of titanium, while the step of forming the second layer includes sputtering from a titanium target in an ambient characterized by a nitrogen flow where the nitrogen flow is sufficient to cause formation of a nitride on the target, to produce a layer of titanium nitride. As another alternative, the steps may be performed to produce a first layer of titanium nitride, and a second layer consisting essentially of titanium.